This invention relates to a method of continuously casting steels, such as low-carbon steels, high-carbon steels, stainless steels and alloy steels, and more particularly to a method of casting slab by width changing technique during casting.
Conventional molds for continuously casting steels are generally made of copper or copper alloy having high thermal conductivity. It is considered critical to form a protective coating over the entire inner surface of the copper or copper alloy base body of the mold to be exposed to molten steel and solidified steel, thereby avoiding the direct contact of the steel with the copper or copper alloy base material. The mold is regarded as unserviceable when the protective coating has been worn away by casting operation to expose the copper or copper alloy base material to a certain extent.
Molds recently introduced into use for continuously casting slab comprise a pair of long side mold pieces (or broad face mold pieces) and a pair of short side mold pieces (or narrow face mold pieces) held therebetween. (The long side mold pieces and short side mold pieces will hereinafter be referred to as "long mold pieces" and "short mold pieces", respectively.) When required, the short mold pieces are slidingly moved longitudinally of the long mold pieces during casting operation to change the width of slab without interrupting the operation. With this technique, however, the sliding contact of the short mold pieces with the long mold pieces produces horizontal scratches in the inner surfaces of the long mold pieces to shorten the life of the mold. This will be described more specifically with reference to FIG. 1 which shows a mold comprising a pair of opposed long mold pieces 1 and a pair of short mold pieces 2 held therebetween. When the short mold pieces 2, as released from the mold clamping force, are slidingly moved toward or away from each other a distance of l.sub.1 between the position defining the maximum width AD of the slab and the position defining the minimum width BC of the slab, extraneous solids (such as solidified steel fragments and solidified vitreous or like lubricant) biting in between the short mold pieces 2 and the long mold pieces 1 create horizontal scratches 3 as shown in FIG. 2. Such scratches occur especially markedly near the mold level. Since the width of the slab is changed considerably frequently in some production processes, the degree or depth of horizontal scratches is by no means negligible. For example, when the lateral movement of the short mold pieces for increasing the slab width creates deep horizontal scratches in the inner surfaces of the long mold pieces, the molten steel moving vertically will encounter increased resistance. Consequently it is even likely that the mold must be replaced in its entirety in a short period of use although flawless except the scratches.
To prevent horizontal scratching due to biting of extraneous solids, it has been proposed or practiced, for example, to ingeniously shape the opposite end faces of the short mold pieces, to supply an antifriction agent to the end faces or to cause the end faces to retain such agent. However, the mold pieces undergo deformation due to thermal strain, contraction, etc. when exposed to the heat of hot molten steel, and such deformation takes place markedly at the level of the molten steel, resulting in the likelihood that a clearance will be formed locally between the end face of the short mold piece and the long mold piece. Once such a clearance occurs in the vicinity of the level of the molten steel, the molten steel flowing in this region ingresses into the clearance and solidifies. When the short mold piece is moved with the solidified steel portion held between the end face of the short mold piece and the long mold piece, the solid steel portion scratches the inner mold side surface of the long mold piece as indicated at 3. To prevent the ingress of molten steel into the clearance, therefore, it has heretofore been necessary to clamp the short mold piece with high pressure and thereby minimize the clearance, but the high clamping pressure is likely to entail another fault such as damage to the mold.